OUR planet is like a bad cake in a cosmic baking contest. On inspection of the first slice, the judges might say its layering is quite neat. The crunchy crust sits on a solid-but-squidgy mantle, which wraps around a gooey outer core. But cut another slice and they will soon see that something has gone awry. Looming inside the neat layers are two giant, messy lumps.
These two blobs are colossal. They are the size of continents, covering almost a third of the boundary between the core and the mantle. We also know that they are hotter than their surroundings. But everything else about these blobs is mysterious, from what they are made of and where they came from to how they affect our planet today.
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The quest to understand them has so far verged on the quixotic. Geologists and planetary scientists are pursuing it with vigour, however, because the blobs are likely to be guarding some serious secrets. We are scrambling to get a better picture of these shadowy underworld titans, not least how ancient they are.
That is important because if they turn out to be geologically youthful, it would suggest we are living through a special epoch. There must be something particularly strange going on down there, to produce such giant oddities. Whereas 鈥渋f these things are truly ancient鈥�, says , Davis, 鈥渋t tells us something about how our planet formed鈥�. And they might even surprise us with an answer to a bigger question, one that goes beyond parochial concerns about our own planet.
Since the late 19th century, geologists have used vibrations called seismic waves, normally generated by earthquakes, to map the interior of our planet. These waves move slowly in less dense and rigid rock, but faster through more tightly packed matter. After studying their speed in countless rock types, geoscientists sent seismic waves through Earth to see the composition of its internal structure: a solid inner core, surrounded by a liquid outer core, which sloshes molten iron and nickel around to generate its magnetic field. On top of this is the mostly solid mantle, the bulk of Earth鈥檚 interior. Capping all this is the crust, an amalgam of rocks that have been erupted, broken up, squashed together and pulled apart. This is what you learned about at school.
But what you may not know is that, in the 1980s, seismic waves , making up . These lumps sit on top of the liquid core, one below the Pacific, one beneath Africa. As wide as ocean basins, they also seem to rise almost 1000 kilometres vertically, into the mantle. They are uneven and misshapen, like the waxy blobs of a lava lamp. But right from the get-go, the questions of what they are doing there, and how they got there, have confounded Earth scientists.
It is even hard to know what to call them. When seismic waves hit the blobs, they slow down. This earned them the name 鈥渓arge low-shear-velocity provinces鈥�, a clumsy collection of words. 鈥淚t鈥檚 not an acronym you can easily say,鈥� says , a planetary scientist at Washington University in St Louis, Missouri. Some call them superplumes. Byrne insists 鈥渂lob is fine鈥�.
Most of what we know about these blobs is through seismology, but seismology has its flaws. Temperature changes the density and rigidity of a rock, but so does its composition. 鈥淚t鈥檚 really hard to tell the difference between the two,鈥� says , a geophysicist at the University of California, Berkeley. Most agree that the blobs are probably hotter than the surrounding mantle, but it is hard to tell if they are made of the same stuff, with lots of iron in them, or if they are packed with other minerals.
Rise and fall
The simplest explanation is that they are made of the same material as the mantle, and are just hotter. If so, their presence may be a result of the disintegration of Pangaea, Earth鈥檚 most recent supercontinent, which formed around 330 million years ago and started breaking up around 200 million years ago. Continents are part of the planet鈥檚 outer shell, made of crust and some upper material in the mantle. As Pangaea broke into tectonic jigsaw pieces, prominent subduction zones 鈥� deep wounds that allow one tectonic plate to descend beneath another 鈥� opened up. For the past 250 million years, defunct chunks of tectonic plates, called slabs, have been descending into the lower mantle. Since the insides of our planet are hotter, the blobs might simply be the warm spots on the core-mantle boundary that aren鈥檛 receiving any of this cooler falling material.
Then again, the simplest explanation isn鈥檛 always the correct one. There is also a chance these blobs aren鈥檛 just hotter, but are also made of different stuff to the rest of the mantle. If so, where they came from is a mystery. And the key to solving the mystery lies in their density, which determines what rises and falls, and gives clues about temperature and chemical composition. 鈥淒ensity is kind of the holy grail in this debate,鈥� says , a seismologist at Royal Holloway, University of London.
Working separately, Lau and Koelemeijer have both been trying to figure how dense these blobs are. In 2017, using GPS sensors to measure tidal changes to the shape of the crust caused by the blobs, Lau and her colleagues estimated the blobs to be fairly dense. But that same year, Koelemeijer and her colleagues used a type of seismic wave sensitive to deep mantle structures, to study where the blobs sit in relation to the core. They were always elevated above the rest of the core, hinting that they were less dense than the surroundings.
The two approaches 鈥渨ere showing us conflicting results鈥�, says Koelemeijer. To crack the case, the researchers decided to team up. Early results from their new work indicating that the blobs may be mostly light 鈥� perhaps bundles of hot, buoyant mantle plumes 鈥� but with dense plant-like roots. But until the results are published, they don鈥檛 want to speculate about what this could mean for the blobs鈥� origins.
Another important conundrum is the age of these blobs. 杏吧原创s examined lava spewed by oceanic volcanoes powered by the blobs (see 鈥淪haping Earth鈥�), finding odd chemistry. Some of this volcanic material seems like it 鈥渉asn鈥檛 ever erupted at the surface of the planet鈥�, says Lau. This includes radioactive elements dating back to the first 50 to 100 million years of Earth鈥檚 life, stuff you won鈥檛 find in younger rocks. 鈥淭hat鈥檚 a very strong argument to say there鈥檚 something really ancient down there,鈥� says Mukhopadhyay.
If so, that would go against the idea that plate tectonics caused the blobs. Plate tectonics began at least 3 billion years ago, but we don鈥檛 know exactly when it started. If the blob matter is truly primordial, even older than the advent of plate tectonics, then where else could it have come from? One option is that this material crystallised deep within the molten soup that was the very young Earth, remaining there since.
A more intriguing suggestion, which has been gaining interest in recent years, is that the blobs come from elsewhere in the solar system.
Around 4.5 billion years ago, when Earth was just an infant, an object the size of Mars, known as Theia, is thought to have slammed into the planet. This giant impact sent molten matter screaming into orbit around our magma-covered world, material that coalesced to form the moon. This idea of how the moon formed has been around since the 1970s, and remains the leading hypothesis. In recent years, however, some have taken it further, wondering if Theia may also be the origin of the blobs. Segments of Theia鈥檚 corpse could have been preserved on the fringes of Earth鈥檚 core for the past 4.5 billion years.
If that鈥檚 right, it would solve the origins of the Earth blobs and settle the debate over how the moon formed in one fell swoop. Except that is a tricky thing to prove. For one, Theia has been destroyed, so we can鈥檛 take samples to compare with the lava created by the blobs鈥� mantle plumes. Another issue arises when trying to virtually reproduce the giant, primordial impact. Chemical analyses of lunar material scooped up during the Apollo era suggest that the moon is mostly made of Earth material, but simulations of the giant primordial impact create a moon mostly made from Theia. A you get something geologically closer to the real moon聽if Theia hit a magma-ocean-covered Earth, but it still isn鈥檛 a perfect replication of reality.
There are various ways we could yet get a better understanding. If Earth blobs truly are primordial, then ancient radioactive elements would give off a unique neutrino signature that, hypothetically, could be detected at the planet鈥檚 surface. But that would need the right sort of detectors placed at the perfect spots, and we aren鈥檛 there yet.
Most scientists hope to do more with the tools they already have 鈥� seismology, chief among them. Most seismometers, the devices that detect seismic waves, are on land, which makes up less than a third of Earth鈥檚 surface area. The oceans, on the other hand, are 鈥渙ne massive blind spot that global seismology is yet to really improve upon鈥�, says Lau. , or vast arrays of sea-floor seismometers that can peer into the planet in considerable detail, are starting to fix that. This sort of research is showing our pair of mystery objects 鈥渘ot as two massive blobs, necessarily鈥�, says Koelemeijer, 鈥渂ut much patchier with more details.鈥�
The plot thickened last year when , a doctoral student at Arizona State University, presented intriguing new results at , held online. According to a combination of his colleagues鈥� prior work and Yuan鈥檚 computer simulations, after Theia鈥檚 collision with our planet 4.5 billion years ago, much of the upper segment of Earth was liquefied, and Theia was largely obliterated. About 20 per cent of Theia鈥檚 mantle punched through to Earth鈥檚 lower mantle, a solid layer that for the most part didn鈥檛 join in with the sloshing molten world above. Yuan鈥檚 argument is that there, below that shield, Theia鈥檚 mantle shards remain, surviving to this very day.
That may sound far-fetched, but it would tally with the hints of primordial matter in some of the lava driven onto the planet鈥檚 surface by the blobs鈥� plumes. And there might be ways to test Yuan鈥檚 hypothesis.
According to Yuan, samples of the moon鈥檚 crust offer additional clues. A team of his colleagues聽 of , and found that they suggest the lunar mantle 鈥� a stand-in for Theia鈥檚 mantle 鈥� has a preponderance of dense iron oxide. That suggests the blobs are denser than Earth鈥檚 mantle.
If so, that may explain why they still exist today: instead of being swept up by the mantle鈥檚 currents and blended into it, their high density let them sink to the base of the mantle and become stubbornly stuck there, to this day. Subducting plates may be influencing the location and composition of the blobs today, but perhaps Theia gave birth to them. That would have been a sight to behold, says Yuan. 鈥淚t鈥檚 beyond my imagination.鈥�
Shaping Earth
Two vast blobs of anomalous material in Earth鈥檚 mantle (see main story) are, geologically speaking, alive. This layer of our planet is populated by towering streaks of superheated material that rise to inflict prolonged, island-making, continent-tearing and occasionally climate-changing volcanism on the surface.
Plume-driven volcanism is unlike any other. It has created chains of islands like the Hawaiian archipelago, home to by far the largest volcanoes on the planet. It played a key role in the dismantling of supercontinents and the creation of ocean basins. And it even contributed to the chaos that unfolded 65 million years ago, unleashing climate-changing volcanic gases while the world reeled from a major asteroid impact.
Although some seem to stand alone, most of these entities, named mantle plumes, appear to sprout from the two blobs. But the way they do this is subject to debate. They might rise up as one continual fountain, or they could appear as many little blobs that together give the illusion of one continuous plume. For now, the main investigation is into where they came from. 鈥淯ntil we know what the blobs are, it seems a bit premature to attribute them to any causal mechanism,鈥� says Paul Byrne at Washington University in St Louis, Missouri.
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